1. Field of the Invention
The present invention relates to semiconductor processing, and more particularly, to a support cylinder used in a thermal processing chamber, such as a Rapid Thermal Processing (RTP) chamber.
2. Description of Related Art
The integrated circuit (IC) market is continually demanding greater memory capacity, faster switching speeds, and smaller feature sizes. One of the major steps the industry has taken to address these demands is to change from batch processing multiple substrates, such as silicon wafers, in large furnaces to processing single substrates in small reaction chambers.
Today, engineers continually strive to increase the throughput of semiconductor substrates while increasing semiconductor yield. The semiconductor substrates referred to herein typically include semiconductor wafers for ultra-large scale integrated (ULSI) circuits.
Generally, there are four basic processes performed in such reaction chambers, namely layering, patterning, doping, and thermal processing. Thermal processing refers to several different processes, including rapid thermal processing (RTP), rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN).
In one RTP process, wafers are loaded into a processing chamber at a temperature of several hundred ° C. in a nitrogen (N2) gas ambient atmosphere. The temperature of the wafer is ramped to reaction conditions, typically at a temperature in the range of about 850° C. to 1200° C. The temperature is raised using a large number of heat sources, such as tungsten halogen lamps, which radiatively heat the wafer. Reactive gases may be introduced before, during, or after the temperature ramp. For example, oxygen may be introduced for growth of silicon dioxide (SiO2).
The uniformity of the thermal process over the surface of the semiconductor substrate during thermal processing is critical to producing uniform semiconductor devices. For example, in the particular application of complementary metal-oxide-semiconductor (CMOS) gate dielectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dielectrics are critical parameters that influence the overall device performance and semiconductor yield. Currently, CMOS devices are being made with dielectric layers that are only 60-80 Å (10−10 m) thick and for which thickness uniformity must be held within a few percent. This level of uniformity requires that temperature variations across the substrate during high temperature processing cannot exceed a few degrees Celsius (° C.). Therefore, techniques that minimize temperature non-uniformity are very important.
Temperature uniformity provides uniform process variables over the substrate (e.g., layer thickness, resistivity, and etch depth) for various process steps including film deposition, oxide growth and etching. In addition, temperature uniformity in the substrate is necessary to prevent thermal stress-induced wafer damage, such as warpage, defect generation, and slip. This type of damage is caused by thermal gradients which are minimized by temperature uniformity. The wafer often cannot tolerate even small temperature differentials during high temperature processing. For example, if the temperature differential is allowed to rise above 1-2° C./cm at 1200° C., the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass. To achieve that level of temperature uniformity, reliable real-time, multi-point temperature measurements are necessary for closed-loop temperature control.
One way of achieving temperature uniformity is by rotating the substrate during processing. This removes the temperature dependence along the azimuthal degree-of-freedom. Because the central axis of the substrate, perpendicular to wafer's surface, is collinear with the axis of rotation 128, all points along any annulus of the wafer (at any arbitrary radius) are exposed to the same amount of illumination. By providing a number of pyrometers and a feedback system, even the remaining radial temperature dependence can be substantially removed, and acceptable temperature uniformity achieved and maintained across the entire substrate.
One example of a type of mechanical rotation system in use today is shown in FIG. 1. This type of system is similar to those used and sold by APPLIED MATERIALS, Inc., of Santa Clara, Calif., U.S.A. Certain details of such systems are provided in U.S. Pat. No. 5,155,336, entitled “Rapid Thermal Heating Apparatus and Method”, issued Oct. 13, 1992, assigned to the assignee of the present invention, and incorporated herein by reference. In this type of mechanical rotation system, the substrate support is rotatably mounted on a bearing assembly that is, in turn, coupled to a vacuum-sealed drive assembly. For example, FIG. 1 depicts a partial cross-sectional view of such a system 100. A wafer 102 is placed on an edge ring 104, which is in turn friction-fit on a cylinder 106. The cylinder 106 sits on a ledge of an upper bearing race 108 which is magnetic. The upper bearing race 108 is disposed within well 110 and is supported on a number of ball bearings 121 (only one of which is shown), relative to a lower bearing race 118. The lower bearing race 118 is mounted on a chamber bottom 120. A water-cooled reflector 124 is positioned on the chamber bottom 120 as part of a temperature measuring system. The temperature measuring system relies on a black body or reflector cavity 122 formed between the reflector 124, the wafer 102, the edge ring 104 and the cylinder 106 to accurately measure the temperature of the wafer 102. Further details of such a black body or reflector cavity can be found in Applicants' U.S. Pat. Nos. 6,174,080; 6,007,241; 6,406,179; 6,374,150; 6,226,453; or 6,183,130, all of which are incorporated herein by reference. The temperature measuring system typically includes a number of pyrometers embedded in the chamber bottom 120.
A magnet 114 is located adjacent the portion of chamber bottom 120 opposite the upper magnetic bearing race 108. The magnet 114 is mounted on a motor-driven magnet ring 116. The magnet 114 is magnetically coupled to the magnetic bearing race 108 through the chamber bottom 120. By mechanically revolving the magnet 114 about the central axis 128, the upper bearing race 108 may be made to rotate as it is magnetically coupled to the magnet 114. In particular, torque is transferred to upper bearing race 108 from the motor-driven magnet ring 116. This rotation of the upper bearing race 108 results in the desired rotation of the wafer 102, which is coupled to the upper bearing race through the cylinder 106 and edge ring 104.
Recent advances have led to a magnetically levitated system to avoid particle generation caused by the contact between the ball bearings 121 and the races as well as from the use of lubrication for the bearing system. Such a magnetically levitated system is described in U.S. Pat. No. 6,157,106 to Tietz et al., which is also incorporated herein by reference.
As mentioned above, precise temperature measurement by the temperature measurement system is a critical process parameter. Therefore, it is important to thermally isolate the temperature measurement system and the upper bearing race 108 from radiation generated by the heat source (not shown). Accordingly, the support cylinder 106 is typically made from ceramic or quartz materials due to their excellent thermal insulation properties. One range of values of conductivity that has been found appropriate is about 1.5 to about 2.5 (J-kg-m)/(m2-sec-° C.). In addition, ceramic and quartz are thermally stable and inert to chemicals typically used in thermal processing.
However, these materials (ceramics and quartz) are transparent to infrared radiation produced by the heat source. Accordingly, without an opaque covering, infrared radiation would pass through the support cylinder and interfere with the highly sensitive temperature measurements being taken by the temperature measurement system. Therefore, the support cylinder is usually coated with a material that is opaque to infrared radiation, such as a polysilicon.
A chemical deposition process, such as Chemical Vapor Deposition (CVD) is typically used to coat the cylinder with the polysilicon. This CVD process is typically performed in a CVD reaction chamber, otherwise known as a bell jar chamber, that includes a heat source. A number of support pins, typically three, are placed on top of the heat source. The cylinder is then placed onto the pins, face down, i.e., the top of the support cylinder, proximate the cylinder makes contact with the support pins. The CVD chamber is then sealed and a gas from which the polysilicon is formed, is pumped into the reaction chamber. The chamber is heated and a layer of polysilicon is subsequently formed on the support cylinder, as is well understood in the art. The deposition rate of the polysilicon onto the support cylinder is about 2.3 μm/min at 1100° C.
This process worked relatively well for support cylinders used for 200 mm semiconductor wafers. However, as the industry has moved to 300 mm semiconductor wafers, several drawbacks have been encountered with the existing cylinder and its method of manufacture.
One such drawback is a non-uniform layer of silicon being applied to the cylinder. Because of the large size or volume of the 300 mm support cylinder, and the good heat insulation properties of the cylinder, it is difficult to uniformly heat the cylinder within the CVD chamber. Indeed, there is a lack of understanding on how to uniformly heat quartz. Heating a thin (˜0.3″), tall (1″) quartz cylinder (OD 13″) to a uniform temperature using a 1D heat source (such as the susceptor) is difficult, so is the ability to uniformly coat the cylinder since the process is temperature dependent. This non-uniform heating of the support cylinder results in a non-uniform layer of polysilicon being formed on the surface of the support cylinder, as shown in an exaggerated cross-sectional view of the cylinder in FIG. 2A. This non-uniform layer of polysilicon may compromise the infrared opacity of the cylinder. Indeed, some areas 204 of the cylinder may have thick polysilicon layer, while other areas 202 have no polysilicon layer at all. As mentioned above, if the cylinder were to allow infrared radiation to penetrate the reflector cavity 122 (FIG. 1), false temperature measurements may be taken leading to imprecise temperature control and ultimately defective semiconductor devices and low semiconductor yield.
Another drawback is cracking of the polysilicon layer, as shown in FIG. 2B, which is a second exaggerated cross-sectional view of the support cylinder. Quartz and polysilicon have different coefficients of thermal expansion, and, therefore, heat and cool at different rates. The coefficients of thermal expansion of quartz and polysilicon are 0.5 vs. 3.8, room temperature to 1000 C, (ppm/° C.) or (10^−6 in/in/° C.).
When the current polysilicon layer cools at a different rate to that of the quartz, it forms cracks 206 in the polysilicon 204. To the naked eye, these cracks 206 in the polysilicon cause a snakeskin like appearance on the surface of the cylinder. Cylinders having this snakeskin like appearance are immediately rejected as defective parts. Accordingly, the cost of producing acceptable cylinders is high.
Another drawback of this non-uniform heating of the support cylinder is that exaggerated growth of the polysilicon may occur at some point of the support cylinder and not at others. This exaggerated growth typically takes the form of dendrites, protrusions, or nodules, as shown in a third exaggerated cross-sectional view of the support cylinder shown in FIG. 2C. As the edge ring 104 (FIG. 1) rests on the cylinder, the dendrites or nodules 208 may cause the edge ring to sit or rest improperly, i.e., not flush, on top of the support cylinder. This may lead to instability of the edge ring on the cylinder and/or nonsymmetrical or eccentric rotation of the wafer. In addition, the formation of the dendrites may affect the height of the wafer from the heat source or lamps. This may negatively impact the temperature that each point on the wafer is exposed to and/or the temperature measurements taken by the pyrometers 126 (FIG. 1). In addition, the dendrites or nodules, may compromise the reflector cavity 122 (FIG. 1) by allowing stray infrared radiation to pass between the edge ring 104 (FIG. 1) and the support cylinder 106 (FIG. 1). In other words, the dendrites may cause an imprecise fit between the cylinder and the edge ring, and/or compromise the thermal isolation of the edge ring from the cylinder. These drawbacks could ultimately lead to defective semiconductor devices and low semiconductor yield.
Accordingly, a cylinder that is opaque to infrared radiation, is easy to manufacture, has a uniform polysilicon layer, is free of dendrites and/or nodules, and does not have a polysilicon layer that cracks, would be highly desirable.